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    <meta charset="UTF-8">
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    <title>Document</title>
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    <script>
        function fn(a){
            //如果a有值，不是null,就||后面白写
            // 如果a是null 
            a = a || 0
            return a + 100 
        }
        console.log(fn(1))//101
        console.log(fn())//NaN
    </script>
</body>
</html>